Procedimiento para el proporcionamiento de las

Five types of Portland Cement have been identified by

the American Society For Testing and Materials. The cement

selected for a LLRW repository depends on the site

environmental conditions and structural requirements. Table

5 indicates the major components of Portland Cement and the

corresponding type as specified by industry.

The proportion of compounds used in Portland Cement

determines its type. Tricalcium silicate (C3S) provides

early strength to the concrete; however, in the curing

process, C3S releases a considerable amount of calcium

hydroxide. Calcium hydroxide protects reinforcing steel from corrosion but increase the susceptibility of sulfate attack. Dicalcium silicate (CoS) cures slowly and is not suitable

for structures requiring early strength. Due to its slow

hydration rate, the rate of heat generation is small and cracking due to thermal expansion is minimized. Tricalcium

aluminate (C3A) reacts rapidly and has a rate of heat

liberation approximately twice that of C3S. C3A provides

high early strength and accounts for much of the shrinkage

in cement. Tetracalcium aluminate-ferrite (C^AF) provides little strength, heat liberation and volume change. C^AF is

much more resistant to sulfate attack.

Type I cement (Table 5) is a general purpose cement. This cement is used when concrete is not subject to sulfate attack or to an excessive temperature rise due to the heat

88 Type of Cement Compound* I Standard 45 II Moderate Heat III High Early Strength 53 IV Low Heat 28 V Sulfate Resi sting Tricalcium Silicate (C3S) 44 38 Dicalcium Silicate (C2S) 27 31 19 49 43 Tricalcium Aluminate (C3A) 11 5. 11 4 4 Tetracalcium Aluminate-Ferrite (C4AF) 8 13 9 12 9 Miscellaneous 9 7 8 7 6 * A represents A1203 C represents CaO F represents Fe203 S represents 3^02 C3S represents (CaO)3S^02

Table 5. Different types of Portland Cement, values in

percent (from Illinois Department of Nuclear

Type II cement is used where there is moderate exposure to sulfate attack or where moderate heat of hydration is permissible. The strength of this cement exceeds Type I

strength after 90 days.

Type III cement provides high strength earlier than Type I and Type II . Due to the pronounced expansion and

contraction while setting, cracking of the concrete is

typically a problem.

Type IV cement contains smaller proportions of C3S and C3A and as a result, is weak at 28 days, but exceeds the

strength of Type I after 90 days. Type IV cement has a low

heat of hydration and was developed for massive concrete

applications such as dams.

Type V cement cannot contain more than five percent by

weight of C3A. Type V cement is used where concrete is

exposed to severe sulfate attack from the soil or ground

water with high sulfate content.

Small quantities of air-entraining material can be added to any of the five types of cement described above. Air

entrainment improves the workability of concrete, reduces

water to cement ratio and in proper amounts produces a low- permeability concrete. Deliberate entrainment of air can

produce a paste that is resistant to freeze-thaw cycling provided sufficient hydration has occurred before the cement

is allowed to freeze while saturated. Slag and pozzolan are

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II, and V cements are suitable materials for the

construction of LLRW facilities.

Concrete Composition

The quality, strength, and expected performance of concrete is determined by the proportion of water, cement,

and aggregate. For high strength concrete, the water/cement ratio should be less than 0.4 or as low as 0.3 (Mackenzie et al.f 1986). In addition, the aggregate/cement paste ratio is equally important. In order to obtain a dense concrete, the void space left unfilled must be kept as low as

possible. In general, the fine aggregate (sand) is

approximately 35-40% by volume of the total aggregate when the maximum size of the coarse aggregate is 3/4 inch. The

proportion of cement used is generally 15% of the aggregate

(by weight or volume). In general, concrete strength is not greatly improved by using a higher proportion of cement; however, decreasing the amount can have adverse consequences since the aggregates will begin to touch each other instead

of being surrounded by cement paste.

Hydraulic Properties

The porosity and permeability of concrete are important

considerations. These properties affect water or

radionuclide migration, degradation mechanisms, and the

durability of concrete. Most concrete likely to be used in

LLRW disposal facilities would be air-intrained. This type

of concrete provides workability and protection from freeze-

thaw.

The porosity of air-entrained concrete ranges from 11 percent to 17 percent. Adding too much water to the

concrete mix causes bleeding and increased porosity. In

newly mixed concrete the porosity may vary from 30 to 40

percent. Effective diffusion coefficients for contaminant

migration in concrete pores range from lO"-'-^ to 10"^ cm^/sec

(Shuman et al.. 1988), depending on the porosity, pore-size

distribution and free water content.

The permeability of a material measures the ability of a gas or liguid to move through it under a pressure gradient. Excluding construction defects, the permeability of concrete

is a function of the permeability of the cement paste and

the aggregate and their bond. A common value for concrete

permeability is lO"-'--'- cm/s.

The permeability of concrete is a major factor affecting the corrosion of reinforcing steel that is embedded in the concrete matrix. A low water to cement ratio, along with well-graded coarse and fine aggregates, produces a concrete

which is less-permeable and more resistant to degradation

processes. in LLRW facilities where the concrete may

contact more than moderate chloride concentrations in the

soil or water, the water/cement ratio should be less than or

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Concrete Degradation Mechanisms

In order to address the various degradation mechanisms of engineered barriers, a well formulated definition of failure is needed. An explicit definition of failure for the disposal facility is given in 10 CFR 61, however,

several choices of engineered barriers may meet the needs. Suitability of a particular type of engineered barrier

depends on site specific characteristics, however, the

choice may be influenced by available data. Based on this rationale, Otis and Cerven (1987) define barrier failure in

context of structural and radionuclide containment failure.

The two definitions are as follows:

o An engineered barrier has failed if its structural

component has lost 50% of its original strength

within the desired lifetime of the unit,

o An engineered barrier has failed if it no longer provides resistance to the movement of radioactive

material greater than that of the surrounding geologic

medium alone.

The durability of concrete and corrosion of steel

reinforcement have been studied for many years. Those

factors that reduce the long-term integrity of concrete are

reasonably well understood. There is, however, little